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Network Working Group A. Melnikov, Ed.
Request for Comments: 4422 Isode Limited
Obsoletes: 2222 K. Zeilenga, Ed.
Category: Standards Track OpenLDAP Foundation
June 2006
Simple Authentication and Security Layer (SASL)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Simple Authentication and Security Layer (SASL) is a framework
for providing authentication and data security services in
connection-oriented protocols via replaceable mechanisms. It
provides a structured interface between protocols and mechanisms.
The resulting framework allows new protocols to reuse existing
mechanisms and allows old protocols to make use of new mechanisms.
The framework also provides a protocol for securing subsequent
protocol exchanges within a data security layer.
This document describes how a SASL mechanism is structured, describes
how protocols include support for SASL, and defines the protocol for
carrying a data security layer over a connection. In addition, this
document defines one SASL mechanism, the EXTERNAL mechanism.
This document obsoletes RFC 2222.
Table of Contents
1. Introduction ....................................................3
1.1. Document Audiences .........................................4
1.2. Relationship to Other Documents ............................4
1.3. Conventions ................................................5
2. Identity Concepts ...............................................5
3. The Authentication Exchange .....................................6
3.1. Mechanism Naming ...........................................8
3.2. Mechanism Negotiation ......................................9
3.3. Request Authentication Exchange ............................9
3.4. Challenges and Responses ...................................9
3.4.1. Authorization Identity String ......................10
3.5. Aborting Authentication Exchanges .........................10
3.6. Authentication Outcome ....................................11
3.7. Security Layers ...........................................12
3.8. Multiple Authentications ..................................12
4. Protocol Requirements ..........................................13
5. Mechanism Requirements .........................................16
6. Security Considerations ........................................18
6.1. Active Attacks ............................................19
6.1.1. Hijack Attacks .....................................19
6.1.2. Downgrade Attacks ..................................19
6.1.3. Replay Attacks .....................................20
6.1.4. Truncation Attacks .................................20
6.1.5. Other Active Attacks ...............................20
6.2. Passive Attacks ...........................................20
6.3. Re-keying .................................................21
6.4. Other Considerations ......................................21
7. IANA Considerations ............................................22
7.1. SASL Mechanism Registry ...................................22
7.2. Registration Changes ......................................26
8. References .....................................................26
8.1. Normative References ......................................26
8.2. Informative References ....................................27
9. Acknowledgements ...............................................28
Appendix A. The SASL EXTERNAL Mechanism ..........................29
A.1. EXTERNAL Technical Specification ..........................29
A.2. SASL EXTERNAL Examples ....................................30
A.3. Security Considerations ...................................31
Appendix B. Changes since RFC 2222 ...............................31
1. Introduction
The Simple Authentication and Security Layer (SASL) is a framework
for providing authentication and data security services in
connection-oriented protocols via replaceable mechanisms. SASL
provides a structured interface between protocols and mechanisms.
SASL also provides a protocol for securing subsequent protocol
exchanges within a data security layer. The data security layer can
provide data integrity, data confidentiality, and other services.
SASL's design is intended to allow new protocols to reuse existing
mechanisms without requiring redesign of the mechanisms and allows
existing protocols to make use of new mechanisms without redesign of
protocols.
SASL is conceptually a framework that provides an abstraction layer
between protocols and mechanisms as illustrated in the following
diagram.
SMTP LDAP XMPP Other protocols ...
\ | | /
\ | | /
SASL abstraction layer
/ | | \
/ | | \
EXTERNAL GSSAPI PLAIN Other mechanisms ...
It is through the interfaces of this abstraction layer that the
framework allows any protocol to utilize any mechanism. While this
layer does generally hide the particulars of protocols from
mechanisms and the particulars of mechanisms from protocols, this
layer does not generally hide the particulars of mechanisms from
protocol implementations. For example, different mechanisms require
different information to operate, some of them use password-based
authentication, some of then require realm information, others make
use of Kerberos tickets, certificates, etc. Also, in order to
perform authorization, server implementations generally have to
implement identity mapping between authentication identities, whose
form is mechanism specific, and authorization identities, whose form
is application protocol specific. Section 2 discusses identity
concepts.
It is possible to design and implement this framework in ways that do
abstract away particulars of similar mechanisms. Such a framework
implementation, as well as mechanisms implementations, could be
designed not only to be shared by multiple implementations of a
particular protocol but to be shared by implementations of multiple
protocols.
The framework incorporates interfaces with both protocols and
mechanisms in which authentication exchanges are carried out.
Section 3 discusses SASL authentication exchanges.
To use SASL, each protocol (amongst other items) provides a method
for identifying which mechanism is to be used, a method for exchange
of mechanism-specific server-challenges and client-responses, and a
method for communicating the outcome of the authentication exchange.
Section 4 discusses SASL protocol requirements.
Each SASL mechanism defines (amongst other items) a series of
server-challenges and client-responses that provide authentication
services and negotiate data security services. Section 5 discusses
SASL mechanism requirements.
Section 6 discusses security considerations. Section 7 discusses
IANA considerations. Appendix A defines the SASL EXTERNAL mechanism.
1.1. Document Audiences
This document is written to serve several different audiences:
- protocol designers using this specification to support
authentication in their protocol,
- mechanism designers that define new SASL mechanisms, and
- implementors of clients or servers for those protocols that
support SASL.
While the document organization is intended to allow readers to focus
on details relevant to their engineering, readers are encouraged to
read and understand all aspects of this document.
1.2. Relationship to Other Documents
This document obsoletes RFC 2222. It replaces all portions of RFC
2222 excepting sections 7.1 (the KERBEROS_IV mechanism), 7.2 (the
GSSAPI mechanism), 7.3 (the SKEY mechanism). The KERBEROS_IV and
SKEY mechanisms are now viewed as obsolete and their specifications
provided in RFC 2222 are Historic. The GSSAPI mechanism is now
separately specified [SASL-GSSAPI].
Appendix B provides a summary of changes since RFC 2222.
1.3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119].
Character names in this document use the notation for code points and
names from the Unicode Standard [Unicode]. For example, the letter
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
Note: a glossary of terms used in Unicode can be found in [Glossary].
Information on the Unicode character encoding model can be found in
[CharModel].
In examples, "C:" and "S:" indicate lines of data to be sent by the
client and server, respectively. Lines have been wrapped for
improved readability.
2. Identity Concepts
In practice, authentication and authorization may involve multiple
identities, possibly in different forms (simple username, Kerberos
principal, X.500 Distinguished Name, etc.), possibly with different
representations (e.g., ABNF-described UTF-8 encoded Unicode character
string, BER-encoded Distinguished Name). While technical
specifications often prescribe both the identity form and
representation used on the network, different identity forms and/or
representations may be (and often are) used within implementations.
How identities of different forms relate to each other is, generally,
a local matter. In addition, the forms and representations used
within an implementation are a local matter.
However, conceptually, the SASL framework involves two identities:
1) an identity associated with the authentication credentials
(termed the authentication identity), and
2) an identity to act as (termed the authorization identity).
SASL mechanism specifications describe the credential form(s) (e.g.,
X.509 certificates, Kerberos tickets, simple username/password) used
to authenticate the client, including (where appropriate) the syntax
and semantics of authentication identities carried in the
credentials. SASL protocol specifications describe the identity
form(s) used in authorization and, in particular, prescribe the
syntax and semantics of the authorization identity character string
to be transferred by mechanisms.
The client provides its credentials (which include or imply an
authentication identity) and, optionally, a character string
representing the requested authorization identity as part of the SASL
exchange. When this character string is omitted or empty, the client
is requesting to act as the identity associated with the credentials
(e.g., the user is requesting to act as the authentication identity).
The server is responsible for verifying the client's credentials and
verifying that the identity it associates with the client's
credentials (e.g., the authentication identity) is allowed to act as
the authorization identity. A SASL exchange fails if either (or
both) of these verifications fails. (The SASL exchange may fail for
other reasons, such as service authorization failure.)
However, the precise form(s) of the authentication identities (used
within the server in its verifications, or otherwise) and the precise
form(s) of the authorization identities (used in making authorization
decisions, or otherwise) are beyond the scope of SASL and this
specification. In some circumstances, the precise identity forms
used in some context outside of the SASL exchange may be dictated by
other specifications. For instance, an identity assumption
authorization (proxy authorization) policy specification may dictate
how authentication and authorization identities are represented in
policy statements.
3. The Authentication Exchange
Each authentication exchange consists of a message from the client to
the server requesting authentication via a particular mechanism,
followed by one or more pairs of challenges from the server and
responses from the client, followed by a message from the server
indicating the outcome of the authentication exchange. (Note:
exchanges may also be aborted as discussed in Section 3.5.)
The following illustration provides a high-level overview of an
authentication exchange.
C: Request authentication exchange
S: Initial challenge
C: Initial response
<additional challenge/response messages>
S: Outcome of authentication exchange
If the outcome is successful and a security layer was negotiated,
this layer is then installed (see Section 3.7). This also applies to
the following illustrations.
Some mechanisms specify that the first data sent in the
authentication exchange is from the client to the server. Protocols
may provide an optional initial response field in the request message
to carry this data. Where the mechanism specifies that the first
data sent in the exchange is from the client to the server, the
protocol provides an optional initial response field, and the client
uses this field, the exchange is shortened by one round-trip:
C: Request authentication exchange + Initial response
<additional challenge/response messages>
S: Outcome of authentication exchange
Where the mechanism specifies that the first data sent in the
exchange is from the client to the server and this field is
unavailable or unused, the client request is followed by an empty
challenge.
C: Request authentication exchange
S: Empty Challenge
C: Initial Response
<additional challenge/response messages>
S: Outcome of authentication exchange
Should a client include an initial response in its request where the
mechanism does not allow the client to send data first, the
authentication exchange fails.
Some mechanisms specify that the server is to send additional data to
the client when indicating a successful outcome. Protocols may
provide an optional additional data field in the outcome message to
carry this data. Where the mechanism specifies that the server is to
return additional data with the successful outcome, the protocol
provides an optional additional data field in the outcome message,
and the server uses this field, the exchange is shortened by one
round-trip:
C: Request authentication exchange
S: Initial challenge
C: Initial response
<additional challenge/response messages>
S: Outcome of authentication exchange with
additional data with success
Where the mechanism specifies that the server is to return additional
data to the client with a successful outcome and this field is
unavailable or unused, the additional data is sent as a challenge
whose response is empty. After receiving this response, the server
then indicates the successful outcome.
C: Request authentication exchange
S: Initial challenge
C: Initial response
<additional challenge/response messages>
S: Additional data challenge
C: Empty Response
S: Outcome of authentication exchange
Where mechanisms specify that the first data sent in the exchange is
from the client to the server and additional data is sent to the
client along with indicating a successful outcome, and the protocol
provides fields supporting both, then the exchange takes two fewer
round-trips:
C: Request authentication exchange + Initial response
<additional challenge/response messages>
S: Outcome of authentication exchange
with additional data with success
instead of:
C: Request authentication exchange
S: Empty Challenge
C: Initial Response
<additional challenge/response messages>
S: Additional data challenge
C: Empty Response
S: Outcome of authentication exchange
3.1. Mechanism Naming
SASL mechanisms are named by character strings, from 1 to 20
characters in length, consisting of ASCII [ASCII] uppercase letters,
digits, hyphens, and/or underscores. In the following Augmented
Backus-Naur Form (ABNF) [RFC4234] grammar, the <sasl-mech> production
defines the syntax of a SASL mechanism name.
sasl-mech = 1*20mech-char
mech-char = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE
; mech-char is restricted to A-Z (uppercase only), 0-9, -, and _
; from ASCII character set.
UPPER-ALPHA = %x41-5A ; A-Z (uppercase only)
DIGIT = %x30-39 ; 0-9
HYPHEN = %x2D ; hyphen (-)
UNDERSCORE = %x5F ; underscore (_)
SASL mechanism names are registered as discussed in Section 7.1.
3.2. Mechanism Negotiation
Mechanism negotiation is protocol specific.
Commonly, a protocol will specify that the server advertises
supported and available mechanisms to the client via some facility
provided by the protocol, and the client will then select the "best"
mechanism from this list that it supports and finds suitable.
Note that the mechanism negotiation is not protected by the
subsequent authentication exchange and hence is subject to downgrade
attacks if not protected by other means.
To detect downgrade attacks, a protocol can allow the client to
discover available mechanisms subsequent to the authentication
exchange and installation of data security layers with at least data
integrity protection. This allows the client to detect changes to
the list of mechanisms supported by the server.
3.3. Request Authentication Exchange
The authentication exchange is initiated by the client by requesting
authentication via a mechanism it specifies. The client sends a
message that contains the name of the mechanism to the server. The
particulars of the message are protocol specific.
Note that the name of the mechanism is not protected by the
mechanism, and hence is subject to alteration by an attacker if not
integrity protected by other means.
Where the mechanism is defined to allow the client to send data
first, and the protocol's request message includes an optional
initial response field, the client may include the response to the
initial challenge in the authentication request message.
3.4. Challenges and Responses
The authentication exchange involves one or more pairs of server-
challenges and client-responses, the particulars of which are
mechanism specific. These challenges and responses are enclosed in
protocol messages, the particulars of which are protocol specific.
Through these challenges and responses, the mechanism may:
- authenticate the client to the server,
- authenticate the server to the client,
- transfer an authorization identity string,
- negotiate a security layer, and
- provide other services.
The negotiation of the security layer may involve negotiation of the
security services to be provided in the layer, how these services
will be provided, and negotiation of a maximum cipher-text buffer
size each side is able to receive in the layer (see Section 3.6).
After receiving an authentication request or any client response, the
server may issue a challenge, abort the exchange, or indicate the
outcome of an exchange. After receiving a challenge, a client
mechanism may issue a response or abort the exchange.
3.4.1. Authorization Identity String
The authorization identity string is a sequence of zero or more
Unicode [Unicode] characters, excluding the NUL (U+0000) character,
representing the identity to act as.
If the authorization identity string is absent, the client is
requesting to act as the identity the server associates with the
client's credentials. An empty string is equivalent to an absent
authorization identity.
A non-empty authorization identity string indicates that the client
wishes to act as the identity represented by the string. In this
case, the form of identity represented by the string, as well as the
precise syntax and semantics of the string, is protocol specific.
While the character encoding schema used to transfer the
authorization identity string in the authentication exchange is
mechanism specific, mechanisms are expected to be capable of carrying
the entire Unicode repertoire (with the exception of the NUL
character).
3.5. Aborting Authentication Exchanges
A client or server may desire to abort an authentication exchange if
it is unwilling or unable to continue (or enter into).
A client may abort the authentication exchange by sending a message,
the particulars of which are protocol specific, to the server,
indicating that the exchange is aborted. The server may be required
by the protocol to return a message in response to the client's abort
message.
Likewise, a server may abort the authentication exchange by sending a
message, the particulars of which are protocol specific, to the
client, indicating that the exchange is aborted.
3.6. Authentication Outcome
At the conclusion of the authentication exchange, the server sends a
message, the particulars of which are protocol specific, to the
client indicating the outcome of the exchange.
The outcome is not successful if
- the authentication exchange failed for any reason,
- the client's credentials could not be verified,
- the server cannot associate an identity with the client's
credentials,
- the client-provided authorization identity string is malformed,
- the identity associated with the client's credentials is not
authorized to act as the requested authorization identity,
- the negotiated security layer (or lack thereof) is not
suitable, or
- the server is not willing to provide service to the client for
any reason.
The protocol may include an optional additional data field in this
outcome message. This field can only include additional data when
the outcome is successful.
If the outcome is successful and a security layer was negotiated,
this layer is then installed. If the outcome is unsuccessful, or a
security layer was not negotiated, any existing security is left in
place.
The outcome message provided by the server can provide a way for the
client to distinguish between errors that are best dealt with by re-
prompting the user for her credentials, errors that are best dealt
with by telling the user to try again later, and errors where the
user must contact a system administrator for resolution (see the SYS
and AUTH POP Response Codes [RFC3206] specification for an example).
This distinction is particularly useful during scheduled server
maintenance periods as it reduces support costs. It is also
important that the server can be configured such that the outcome
message will not distinguish between a valid user with invalid
credentials and an invalid user.
3.7. Security Layers
SASL mechanisms may offer a wide range of services in security
layers. Typical services include data integrity and data
confidentiality. SASL mechanisms that do not provide a security
layer are treated as negotiating no security layer.
If use of a security layer is negotiated in the authentication
protocol exchange, the layer is installed by the server after
indicating the outcome of the authentication exchange and installed
by the client upon receipt of the outcome indication. In both cases,
the layer is installed before transfer of further protocol data. The
precise position upon which the layer takes effect in the protocol
data stream is protocol specific.
Once the security layer is in effect in the protocol data stream, it
remains in effect until either a subsequently negotiated security
layer is installed or the underlying transport connection is closed.
When in effect, the security layer processes protocol data into
buffers of protected data. If at any time the security layer is
unable or unwilling to continue producing buffers protecting protocol
data, the underlying transport connection MUST be closed. If the
security layer is not able to decode a received buffer, the
underlying connection MUST be closed. In both cases, the underlying
transport connection SHOULD be closed gracefully.
Each buffer of protected data is transferred over the underlying
transport connection as a sequence of octets prepended with a four-
octet field in network byte order that represents the length of the
buffer. The length of the protected data buffer MUST be no larger
than the maximum size that the other side expects. Upon the receipt
of a length field whose value is greater than the maximum size, the
receiver SHOULD close the connection, as this might be a sign of an
attack.
The maximum size that each side expects is fixed by the mechanism,
either through negotiation or by its specification.
3.8. Multiple Authentications
Unless explicitly permitted in the protocol (as stated in the
protocol's technical specification), only one successful SASL
authentication exchange may occur in a protocol session. In this
case, once an authentication exchange has successfully completed,
further attempts to initiate an authentication exchange fail.
Where multiple successful SASL authentication exchanges are permitted
in the protocol, then in no case may multiple SASL security layers be
simultaneously in effect. If a security layer is in effect and a
subsequent SASL negotiation selects a second security layer, then the
second security layer replaces the first. If a security layer is in
effect and a subsequent SASL negotiation selects no security layer,
the original security layer remains in effect.
Where multiple successful SASL negotiations are permitted in the
protocol, the effect of a failed SASL authentication exchange upon
the previously established authentication and authorization state is
protocol specific. The protocol's technical specification should be
consulted to determine whether the previous authentication and
authorization state remains in force, or changed to an anonymous
state, or otherwise was affected. Regardless of the protocol-
specific effect upon previously established authentication and
authorization state, the previously negotiated security layer remains
in effect.
4. Protocol Requirements
In order for a protocol to offer SASL services, its specification
MUST supply the following information:
1) A service name, to be selected from registry of "service" elements
for the Generic Security Service Application Program Interface
(GSSAPI) host-based service name form, as described in Section 4.1
of [RFC2743]. Note that this registry is shared by all GSSAPI and
SASL mechanisms.
2) Detail any mechanism negotiation facility that the protocol
provides (see Section 3.2).
A protocol SHOULD specify a facility through which the client may
discover, both before initiation of the SASL exchange and after
installing security layers negotiated by the exchange, the names
of the SASL mechanisms that the server makes available to the
client. The latter is important to allow the client to detect
downgrade attacks. This facility is typically provided through
the protocol's extensions or capabilities discovery facility.
3) Definition of the messages necessary for authentication exchange,
including the following:
a) A message to initiate the authentication exchange (see Section
3.3).
This message MUST contain a field for carrying the name of the
mechanism selected by the client.
This message SHOULD contain an optional field for carrying an
initial response. If the message is defined with this field,
the specification MUST describe how messages with an empty
initial response are distinguished from messages with no
initial response. This field MUST be capable of carrying
arbitrary sequences of octets (including zero-length sequences
and sequences containing zero-valued octets).
b) Messages to transfer server challenges and client responses
(see Section 3.4).
Each of these messages MUST be capable of carrying arbitrary
sequences of octets (including zero-length sequences and
sequences containing zero-valued octets).
c) A message to indicate the outcome of the authentication
exchange (see Section 3.6).
This message SHOULD contain an optional field for carrying
additional data with a successful outcome. If the message is
defined with this field, the specification MUST describe how
messages with an empty additional data are distinguished from
messages with no additional data. This field MUST be capable
of carrying arbitrary sequences of octets (including zero-
length sequences and sequences containing zero-valued octets).
4) Prescribe the syntax and semantics of non-empty authorization
identity strings (see Section 3.4.1).
In order to avoid interoperability problems due to differing
normalizations, the protocol specification MUST detail precisely
how and where (client or server) non-empty authorization identity
strings are prepared, including all normalizations, for comparison
and other applicable functions to ensure proper function.
Specifications are encouraged to prescribe use of existing
authorization identity forms as well as existing string
representations, such as simple user names [RFC4013].
Where the specification does not precisely prescribe how
identities in SASL relate to identities used elsewhere in the
protocol, for instance, in access control policy statements, it
may be appropriate for the protocol to provide a facility by which
the client can discover information (such as the representation of
the identity used in making access control decisions) about
established identities for these uses.
5) Detail any facility the protocol provides that allows the client
and/or server to abort authentication exchange (see Section 3.5).
Protocols that support multiple authentications typically allow a
client to abort an ongoing authentication exchange by initiating a
new authentication exchange. Protocols that do not support
multiple authentications may require the client to close the
connection and start over to abort an ongoing authentication
exchange.
Protocols typically allow the server to abort ongoing
authentication exchanges by returning a non-successful outcome
message.
6) Identify precisely where newly negotiated security layers start to
take effect, in both directions (see Section 3.7).
Typically, specifications require security layers to start taking
effect on the first octet following the outcome message in data
being sent by the server and on the first octet sent after receipt
of the outcome message in data being sent by the client.
7) If the protocol supports other layered security services, such as
Transport Layer Security (TLS) [RFC4346], the specification MUST
prescribe the order in which security layers are applied to
protocol data.
For instance, where a protocol supports both TLS and SASL security
layers, the specification could prescribe any of the following:
a) SASL security layer is always applied first to data being sent
and, hence, applied last to received data,
b) SASL security layer is always applied last to data being sent
and, hence, applied first to received data,
c) Layers are applied in the order in which they were installed,
d) Layers are applied in the reverse order in which they were
installed, or
e) Both TLS and SASL security layers cannot be installed.
8) Indicate whether the protocol supports multiple authentications
(see Section 3.8). If so, the protocol MUST detail the effect a
failed SASL authentication exchange will have upon a previously
established authentication and authorization state.
Protocol specifications SHOULD avoid stating implementation
requirements that would hinder replacement of applicable mechanisms.
In general, protocol specifications SHOULD be mechanism neutral.
There are a number of reasonable exceptions to this recommendation,
including
- detailing how credentials (which are mechanism specific) are
managed in the protocol,
- detailing how authentication identities (which are mechanism
specific) and authorization identities (which are protocol
specific) relate to each other, and
- detailing which mechanisms are applicable to the protocol.
5. Mechanism Requirements
SASL mechanism specifications MUST supply the following information:
1) The name of the mechanism (see Section 3.1). This name MUST be
registered as discussed in Section 7.1.
2) A definition of the server-challenges and client-responses of the
authentication exchange, as well as the following:
a) An indication of whether the mechanism is client-first,
variable, or server-first. If a SASL mechanism is defined as
client-first and the client does not send an initial response
in the authentication request, then the first server challenge
MUST be empty (the EXTERNAL mechanism is an example of this
case). If a SASL mechanism is defined as variable, then the
specification needs to state how the server behaves when the
initial client response in the authentication request is
omitted (the DIGEST-MD5 mechanism [DIGEST-MD5] is an example of
this case). If a SASL mechanism is defined as server-first,
then the client MUST NOT send an initial client response in the
authentication request (the CRAM-MD5 mechanism [CRAM-MD5] is an
example of this case).
b) An indication of whether the server is expected to provide
additional data when indicating a successful outcome. If so,
if the server sends the additional data as a challenge, the
specification MUST indicate that the response to this challenge
is an empty response.
SASL mechanisms SHOULD be designed to minimize the number of
challenges and responses necessary to complete the exchange.
3) An indication of whether the mechanism is capable of transferring
authorization identity strings (see Section 3.4.1). While some
legacy mechanisms are incapable of transmitting an authorization
identity (which means that for these mechanisms, the authorization
identity is always the empty string), newly defined mechanisms
SHOULD be capable of transferring authorization identity strings.
The mechanism SHOULD NOT be capable of transferring both no
authorization identity string and an empty authorization identity.
Mechanisms that are capable of transferring an authorization
identity string MUST be capable of transferring arbitrary non-
empty sequences of Unicode characters, excluding those that
contain the NUL (U+0000) character. Mechanisms SHOULD use the
UTF-8 [RFC3629] transformation format. The specification MUST
detail how any Unicode code points special to the mechanism that
might appear in the authorization identity string are escaped to
avoid ambiguity during decoding of the authorization identity
string. Typically, mechanisms that have special characters
require these special characters to be escaped or encoded in the
character string (after encoding it in a particular Unicode
transformation format) using a data encoding scheme such as Base64
[RFC3548].
4) The specification MUST detail whether the mechanism offers a
security layer. If the mechanism does, the specification MUST
detail the security and other services offered in the layer as
well as how these services are to be implemented.
5) If the underlying cryptographic technology used by a mechanism
supports data integrity, then the mechanism specification MUST
integrity protect the transmission of an authorization identity
and the negotiation of the security layer.
SASL mechanisms SHOULD be protocol neutral.
SASL mechanisms SHOULD reuse existing credential and identity forms,
as well as associated syntaxes and semantics.
SASL mechanisms SHOULD use the UTF-8 transformation format [RFC3629]
for encoding Unicode [Unicode] code points for transfer.
In order to avoid interoperability problems due to differing
normalizations, when a mechanism calls for character data (other than
the authorization identity string) to be used as input to a
cryptographic and/or comparison function, the specification MUST
detail precisely how and where (client or server) the character data
is to be prepared, including all normalizations, for input into the
function to ensure proper operation.
For simple user names and/or passwords in authentication credentials,
SASLprep [RFC4013] (a profile of the StringPrep [RFC3454] preparation
algorithm), SHOULD be specified as the preparation algorithm.
The mechanism SHOULD NOT use the authorization identity string in
generation of any long-term cryptographic keys or hashes as there is
no requirement that the authorization identity string be canonical.
Long-term, here, means a term longer than the duration of the
authentication exchange in which they were generated. That is, as
different clients (of the same or different protocol) may provide
different authorization identity strings that are semantically
equivalent, use of authorization identity strings in generation of
cryptographic keys and hashes will likely lead to interoperability
and other problems.
6. Security Considerations
Security issues are discussed throughout this memo.
Many existing SASL mechanisms do not provide adequate protection
against passive attacks, let alone active attacks, in the
authentication exchange. Many existing SASL mechanisms do not offer
security layers. It is hoped that future SASL mechanisms will
provide strong protection against passive and active attacks in the
authentication exchange, as well as security layers with strong basic
data security features (e.g., data integrity and data
confidentiality) services. It is also hoped that future mechanisms
will provide more advanced data security services like re-keying (see
Section 6.3).
Regardless, the SASL framework is susceptible to downgrade attacks.
Section 6.1.2 offers a variety of approaches for preventing or
detecting these attacks. In some cases, it is appropriate to use
data integrity protective services external to SASL (e.g., TLS) to
protect against downgrade attacks in SASL. Use of external
protective security services is also important when the mechanisms
available do not themselves offer adequate integrity and/or
confidentiality protection of the authentication exchange and/or
protocol data.
6.1. Active Attacks
6.1.1. Hijack Attacks
When the client selects a SASL security layer with at least integrity
protection, this protection serves as a counter-measure against an
active attacker hijacking the connection and modifying protocol data
sent after establishment of the security layer. Implementations
SHOULD close the connection when the security services in a SASL
security layer report protocol data report lack of data integrity.
6.1.2. Downgrade Attacks
It is important that any security-sensitive protocol negotiations be
performed after installation of a security layer with data integrity
protection. Protocols should be designed such that negotiations
performed prior to this installation should be revalidated after
installation is complete. Negotiation of the SASL mechanism is
security sensitive.
When a client negotiates the authentication mechanism with the server
and/or other security features, it is possible for an active attacker
to cause a party to use the least secure security services available.
For instance, an attacker can modify the server-advertised mechanism
list or can modify the client-advertised security feature list within
a mechanism response. To protect against this sort of attack,
implementations SHOULD NOT advertise mechanisms and/or features that
cannot meet their minimum security requirements, SHOULD NOT enter
into or continue authentication exchanges that cannot meet their
minimum security requirements, and SHOULD verify that completed
authentication exchanges result in security services that meet their
minimum security requirements. Note that each endpoint needs to
independently verify that its security requirements are met.
In order to detect downgrade attacks to the least (or less) secure
mechanism supported, the client can discover the SASL mechanisms that
the server makes available both before the SASL authentication
exchange and after the negotiated SASL security layer (with at least
data integrity protection) has been installed through the protocol's
mechanism discovery facility. If the client finds that the
integrity-protected list (the list obtained after the security layer
was installed) contains a stronger mechanism than those in the
previously obtained list, the client should assume that the
previously obtained list was modified by an attacker and SHOULD close
the underlying transport connection.
The client's initiation of the SASL exchange, including the selection
of a SASL mechanism, is done in the clear and may be modified by an
active attacker. It is important for any new SASL mechanisms to be
designed such that an active attacker cannot obtain an authentication
with weaker security properties by modifying the SASL mechanism name
and/or the challenges and responses.
Multi-level negotiation of security features is prone to downgrade
attack. Protocol designers should avoid offering higher-level
negotiation of security features in protocols (e.g., above SASL
mechanism negotiation) and mechanism designers should avoid lower-
level negotiation of security features in mechanisms (e.g., below
SASL mechanism negotiation).
6.1.3. Replay Attacks
Some mechanisms may be subject to replay attacks unless protected by
external data security services (e.g., TLS).
6.1.4. Truncation Attacks
Most existing SASL security layers do not themselves offer protection
against truncation attack. In a truncation attack, the active
attacker causes the protocol session to be closed, causing a
truncation of the possibly integrity-protected data stream that leads
to behavior of one or both the protocol peers that inappropriately
benefits the attacker. Truncation attacks are fairly easy to defend
against in connection-oriented application-level protocols. A
protocol can defend against these attacks by ensuring that each
information exchange has a clear final result and that each protocol
session has a graceful closure mechanism, and that these are
integrity protected.
6.1.5. Other Active Attacks
When use of a security layer is negotiated by the authentication
protocol exchange, the receiver SHOULD handle gracefully any
protected data buffer larger than the defined/negotiated maximal
size. In particular, it MUST NOT blindly allocate the amount of
memory specified in the buffer size field, as this might cause the
"out of memory" condition. If the receiver detects a large block, it
SHOULD close the connection.
6.2. Passive Attacks
Many mechanisms are subject to various passive attacks, including
simple eavesdropping of unprotected credential information as well as
online and offline dictionary attacks of protected credential
information.
6.3. Re-keying
The secure or administratively permitted lifetimes of SASL
mechanisms' security layers are finite. Cryptographic keys weaken as
they are used and as time passes; the more time and/or cipher-text
that a cryptanalyst has after the first use of the a key, the easier
it is for the cryptanalyst to mount attacks on the key.
Administrative limits on a security layer's lifetime may take the
form of time limits expressed in X.509 certificates, in Kerberos V
tickets, or in directories, and are often desired. In practice, one
likely effect of administrative lifetime limits is that applications
may find that security layers stop working in the middle of
application protocol operation, such as, perhaps, during large data
transfers. As the result of this, the connection will be closed (see
Section 3.7), which will result in an unpleasant user experience.
Re-keying (key renegotiation process) is a way of addressing the
weakening of cryptographic keys. The SASL framework does not itself
provide for re-keying; SASL mechanisms may. Designers of future SASL
mechanisms should consider providing re-keying services.
Implementations that wish to re-key SASL security layers where the
mechanism does not provide for re-keying SHOULD reauthenticate the
same IDs and replace the expired or soon-to-expire security layers.
This approach requires support for reauthentication in the
application protocols (see Section 3.8).
6.4. Other Considerations
Protocol designers and implementors should understand the security
considerations of mechanisms so they may select mechanisms that are
applicable to their needs.
Distributed server implementations need to be careful in how they
trust other parties. In particular, authentication secrets should
only be disclosed to other parties that are trusted to manage and use
those secrets in a manner acceptable to the disclosing party.
Applications using SASL assume that SASL security layers providing
data confidentiality are secure even when an attacker chooses the
text to be protected by the security layer. Similarly, applications
assume that the SASL security layer is secure even if the attacker
can manipulate the cipher-text output of the security layer. New
SASL mechanisms are expected to meet these assumptions.
Unicode security considerations [UTR36] apply to authorization
identity strings, as well as UTF-8 [RFC3629] security considerations
where UTF-8 is used. SASLprep [RFC4013] and StringPrep [RFC3454]
security considerations also apply where used.
7. IANA Considerations
7.1. SASL Mechanism Registry
The SASL mechanism registry is maintained by IANA. The registry is
currently available at <http://www.iana.org/assignments/sasl-
mechanisms>.
The purpose of this registry is not only to ensure uniqueness of
values used to name SASL mechanisms, but also to provide a definitive
reference to technical specifications detailing each SASL mechanism
available for use on the Internet.
There is no naming convention for SASL mechanisms; any name that
conforms to the syntax of a SASL mechanism name can be registered.
The procedure detailed in Section 7.1.1 is to be used for
registration of a value naming a specific individual mechanism.
The procedure detailed in Section 7.1.2 is to be used for
registration of a value naming a family of related mechanisms.
Comments may be included in the registry as discussed in Section
7.1.3 and may be changed as discussed in Section 7.1.4.
The SASL mechanism registry has been updated to reflect that this
document provides the definitive technical specification for SASL and
that this section provides the registration procedures for this
registry.
7.1.1. Mechanism Name Registration Procedure
IANA will register new SASL mechanism names on a First Come First
Served basis, as defined in BCP 26 [RFC2434]. IANA has the right to
reject obviously bogus registration requests, but will perform no
review of claims made in the registration form.
Registration of a SASL mechanism is requested by filling in the
following template:
Subject: Registration of SASL mechanism X
SASL mechanism name (or prefix for the family):
Security considerations:
Published specification (recommended):
Person & email address to contact for further information:
Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE)
Owner/Change controller:
Note: (Any other information that the author deems relevant may be
added here.)
and sending it via electronic mail to IANA at <iana@iana.org>.
While this registration procedure does not require expert review,
authors of SASL mechanisms are encouraged to seek community review
and comment whenever that is feasible. Authors may seek community
review by posting a specification of their proposed mechanism as an
Internet-Draft. SASL mechanisms intended for widespread use should
be standardized through the normal IETF process, when appropriate.
7.1.2. Family Name Registration Procedure
As noted above, there is no general naming convention for SASL
mechanisms. However, specifications may reserve a portion of the
SASL mechanism namespace for a set of related SASL mechanisms, a
"family" of SASL mechanisms. Each family of SASL mechanisms is
identified by a unique prefix, such as X-. Registration of new SASL
mechanism family names requires expert review as defined in BCP 26
[RFC2434].
Registration of a SASL family name is requested by filling in the
following template:
Subject: Registration of SASL mechanism family X
SASL family name (or prefix for the family):
Security considerations:
Published specification (recommended):
Person & email address to contact for further information:
Intended usage: (One of COMMON, LIMITED USE, or OBSOLETE)
Owner/Change controller:
Note: (Any other information that the author deems relevant may be
added here.)
and sending it via electronic mail to the IETF SASL mailing list at
<ietf-sasl@imc.org> and carbon copying IANA at <iana@iana.org>.
After allowing two weeks for community input on the IETF SASL mailing
list, the expert will determine the appropriateness of the
registration request and either approve or disapprove the request
with notice to the requestor, the mailing list, and IANA.
The review should focus on the appropriateness of the requested
family name for the proposed use and the appropriateness of the
proposed naming and registration plan for existing and future
mechanism names in the family. The scope of this request review may
entail consideration of relevant aspects of any provided technical
specification, such as their IANA Considerations section. However,
this review is narrowly focused on the appropriateness of the
requested registration and not on the overall soundness of any
provided technical specification.
Authors are encouraged to pursue community review by posting the
technical specification as an Internet-Draft and soliciting comment
by posting to appropriate IETF mailing lists.
7.1.3. Comments on SASL Mechanism Registrations
Comments on a registered SASL mechanism/family should first be sent
to the "owner" of the mechanism/family and/or to the <ietf-
sasl@imc.org> mailing list.
Submitters of comments may, after a reasonable attempt to contact the
owner, request IANA to attach their comment to the SASL mechanism
registration itself by sending mail to <iana@iana.org>. At IANA's
sole discretion, IANA may attach the comment to the SASL mechanism's
registration.
7.1.4. Change Control
Once a SASL mechanism registration has been published by IANA, the
author may request a change to its definition. The change request
follows the same procedure as the registration request.
The owner of a SASL mechanism may pass responsibility for the SASL
mechanism to another person or agency by informing IANA; this can be
done without discussion or review.
The IESG may reassign responsibility for a SASL mechanism. The most
common case of this will be to enable changes to be made to
mechanisms where the author of the registration has died, has moved
out of contact, or is otherwise unable to make changes that are
important to the community.
SASL mechanism registrations may not be deleted; mechanisms that are
no longer believed appropriate for use can be declared OBSOLETE by a
change to their "intended usage" field; such SASL mechanisms will be
clearly marked in the lists published by IANA.
The IESG is considered to be the owner of all SASL mechanisms that
are on the IETF standards track.
7.2. Registration Changes
The IANA has updated the SASL mechanisms registry as follows:
1) Changed the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
registrations to OBSOLETE.
2) Changed the "Published specification" of the EXTERNAL mechanism to
this document as indicated below:
Subject: Updated Registration of SASL mechanism EXTERNAL
Family of SASL mechanisms: NO
SASL mechanism name: EXTERNAL
Security considerations: See A.3 of RFC 4422
Published specification (optional, recommended): RFC 4422
Person & email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for EXTERNAL
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2244] Newman, C. and J. G. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November
1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[ASCII] Coded Character Set--7-bit American Standard Code for
Information Interchange, ANSI X3.4-1986.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version
3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
61633-5), as amended by the "Unicode Standard Annex
#27: Unicode 3.1"
(http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
(http://www.unicode.org/reports/tr28/).
[CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
#17, Character Encoding Model", UTR17,
<http://www.unicode.org/unicode/reports/tr17/>, August
2000.
[Glossary] The Unicode Consortium, "Unicode Glossary",
<http://www.unicode.org/glossary/>.
8.2. Informative References
[RFC3206] Gellens, R., "The SYS and AUTH POP Response Codes", RFC
3206, February 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346, April
2006.
[SASL-GSSAPI] Melnikov, A. (Editor), "The Kerberos V5 ("GSSAPI") SASL
Mechanism", Work in Progress, May 2006.
[UTR36] Davis, M., "(Draft) Unicode Technical Report #36,
Character Encoding Model", UTR17,
<http://www.unicode.org/unicode/reports/tr36/>,
February 2005.
[CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", Work in
Progress.
[DIGEST-MD5] Leach, P., C. Newman, and A. Melnikov, "Using Digest
Authentication as a SASL Mechanism", Work in Progress,
March 2006.
9. Acknowledgements
This document is a revision of RFC 2222 written by John Myers.
This revision is a product of the IETF Simple Authentication and
Security Layer (SASL) Working Group.
The following individuals contributed significantly to this revision:
Abhijit Menon-Sen, Hallvard Furuseth, Jeffrey Hutzelman, John Myers,
Luke Howard, Magnus Nystrom, Nicolas Williams, Peter Saint-Andre, RL
'Bob' Morgan, Rob Siemborski, Sam Hartman, Simon Josefsson, Tim
Alsop, and Tony Hansen.
Appendix A. The SASL EXTERNAL Mechanism
This appendix is normative.
The EXTERNAL mechanism allows a client to request the server to use
credentials established by means external to the mechanism to
authenticate the client. The external means may be, for instance, IP
Security [RFC4301] or TLS [RFC4346] services. In absence of some a
priori agreement between the client and the server, the client cannot
make any assumption as to what external means the server has used to
obtain the client's credentials, nor make an assumption as to the
form of credentials. For example, the client cannot assume that the
server will use the credentials the client has established via TLS.
A.1. EXTERNAL Technical Specification
The name of this mechanism is "EXTERNAL".
The mechanism does not provide a security layer.
The mechanism is capable of transferring an authorization identity
string. If empty, the client is requesting to act as the identity
the server has associated with the client's credentials. If non-
empty, the client is requesting to act as the identity represented by
the string.
The client is expected to send data first in the authentication
exchange. Where the client does not provide an initial response data
in its request to initiate the authentication exchange, the server is
to respond to the request with an empty initial challenge and then
the client is to provide its initial response.
The client sends the initial response containing the UTF-8 [RFC3629]
encoding of the requested authorization identity string. This
response is non-empty when the client is requesting to act as the
identity represented by the (non-empty) string. This response is
empty when the client is requesting to act as the identity the server
associated with its authentication credentials.
The syntax of the initial response is specified as a value of the
<extern-initial-resp> production detailed below using the Augmented
Backus-Naur Form (ABNF) [RFC4234] notation.
external-initial-resp = authz-id-string
authz-id-string = *( UTF8-char-no-nul )
UTF8-char-no-nul = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4
UTF8-1-no-nul = %x01-7F
where the <UTF8-2>, <UTF8-3>, and <UTF8-4> productions are as defined
in [RFC3629].
There are no additional challenges and responses.
Hence, the server is to return the outcome of the authentication
exchange.
The exchange fails if
- the client has not established its credentials via external means,
- the client's credentials are inadequate,
- the client provided an empty authorization identity string and the
server is unwilling or unable to associate an authorization
identity with the client's credentials,
- the client provided a non-empty authorization identity string that
is invalid per the syntax requirements of the applicable
application protocol specification,
- the client provided a non-empty authorization identity string
representing an identity that the client is not allowed to act as,
or
- the server is unwilling or unable to provide service to the client
for any other reason.
Otherwise the exchange is successful. When indicating a successful
outcome, additional data is not provided.
A.2. SASL EXTERNAL Examples
This section provides examples of EXTERNAL authentication exchanges.
The examples are intended to help the readers understand the above
text. The examples are not definitive. The Application
Configuration Access Protocol (ACAP) [RFC2244] is used in the
examples.
The first example shows use of EXTERNAL with an empty authorization
identity. In this example, the initial response is not sent in the
client's request to initiate the authentication exchange.
S: * ACAP (SASL "DIGEST-MD5")
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL")
C: a002 AUTHENTICATE "EXTERNAL"
S: + ""
C: + ""
S: a002 OK "Authenticated"
The second example shows use of EXTERNAL with an authorization
identity of "fred@example.com". In this example, the initial
response is sent with the client's request to initiate the
authentication exchange. This saves a round-trip.
S: * ACAP (SASL "DIGEST-MD5")
C: a001 STARTTLS
S: a001 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL")
C: a002 AUTHENTICATE "EXTERNAL" {16+}
C: fred@example.com
S: a002 NO "Cannot assume requested authorization identity"
A.3. Security Considerations
The EXTERNAL mechanism provides no security protection; it is
vulnerable to spoofing by either client or server, active attack, and
eavesdropping. It should only be used when adequate security
services have been established.
Appendix B. Changes since RFC 2222
This appendix is non-normative.
The material in RFC 2222 was significantly rewritten in the
production of this document.
RFC 2222, by not stating that the authorization identity string was a
string of Unicode characters, let alone character data, implied that
the authorization identity string was a string of octets.
- The authorization identity string is now defined as a string of
Unicode characters. The NUL (U+0000) character is prohibited.
While protocol specifications are responsible for defining the
authorization identity form, as well as the Unicode string syntax
and related semantics, mechanism specifications are responsible
for defining how the Unicode string is carried in the
authentication exchange.
- Deleted "If so, when the client does not send data first, the
initial challenge MUST be specified as being an empty challenge."
The following technical change was made to the EXTERNAL mechanism:
- The authorization identity string is to be UTF-8 encoded.
Note that protocol and mechanism specification requirements have
been significantly tightened. Existing protocol and mechanism
specifications will need to be updated to meet these requirements.
Editors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex,
TW12 2BX, United Kingdom
EMail: Alexey.Melnikov@isode.com
URI: http://www.melnikov.ca/
Kurt D. Zeilenga
OpenLDAP Foundation
EMail: Kurt@OpenLDAP.org
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